1. Field of the Invention
This invention relates to high consistency organosiloxane compositions. More particularly, this invention relates to improved high consistency organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction to yield cured products exhibiting better physical properties, particularly tear strength, relative to prior art compositions.
2. Description of the Prior Art
High consistency organosiloxane elastomer compositions are known in the art. These compositions typically contain at least one polydiorganosiloxane having a viscosity of at least 1000 Pa.s (equivalent to the consistency of a gum), a reinforcing silica filler, an optional silica treating agent to prevent an irreversible thickening referred to as "creping" during processing of the curable composition, and a curing agent. The curing agent for high consistency organosiloxane compositions is typically an organic peroxide. Compositions of this type are cured by heating them above the decomposition temperature of the organic peroxide.
U.S. Pat. No. 4,089,833, which issued to Simpson on May 16, 1978 teaches that high consistency organosiloxane compositions can be cured either by organic peroxides or a platinum-catalyzed hydrosilation reaction. The compositions curable by the hydrosilation reaction include a vinyl-containing polydiorganosiloxane, and, as the curing agent, an organohydrogensiloxane. The two types of polydiorganosiloxanes disclosed in this patent are (1) a polydiorganosiloxane having a viscosity of from 500,000 to 200,000,000 centipoise (500 to 200,000 Pa.s) that contains silicon-bonded lower alkyl and vinyl radicals and from 5 to 50 mole percent, based on the total number of silicon-bonded organic groups, of 3,3,3-trifluoropropyl radicals and (2) a non-halogenated "process aid". The process aid is defined as "a diorganopolysiloxane having a viscosity varying from 100,000 to 10,000,000 centipoise at 25.degree. C. where the organic groups are selected from alkyl radicals, vinyl radicals, phenyl radicals and mixtures thereof, and wherein the vinyl unsaturation varies from 5 to 25 percent". The compositions also include a reinforcing silica filler. The process aid prevents the composition from sticking to or partitioning on processing equipment.
The aforementioned patent issued to Simpson does not distinguish between compositions cured using an organic peroxide and those cured by a platinum-catalyzed hydrosilation reaction. The latter class of compositions are not exemplified.
U.S. Pat. No. 3,671,480, which issued to Wada on June 20, 1972 teaches heat curable organosiloxane elastomer compositions comprising 2 different vinyl-containing polydiorganosiloxanes, a polyorganohydrogensiloxane and a platinum-containing catalyst. One of the two vinyl-containing polymers is a gum having a degree of polymerization greater than 3000 and containing from 0.02 to 0.2 mole percent of vinyl radicals. The second vinyl-containing polymer contains at least 2 mole percent vinyl radicals and has a degree of polymerization less than 100, which encompasses only liquid polydiorganosiloxanes. Two of the four exemplified compositions contain mixtures of a gum and a liquid polydiorganosiloxane as the two vinyl-containing polymers, however none of the exemplified compositions contain two immiscible vinyl-containing polydiorganosiloxanes.
The use of a low molecular weight, vinyl-substituted polydimethylsiloxane "oil" to extend the processing temperature range of a specified class of peroxide curable organosiloxane compositions and the tear resistance of cured elastomers prepared from these compositions is taught in U.S. Pat. No. 4,463,118, which issued on July 31, 1984 to Evans and Razzano. The curable polymer in these compositions is a silarylenesiloxane/diorganosiloxane block copolymer wherein the silarylenesiloxane units contains at least one pair of silicon atoms joined by a arylene hydrocarbon radical. Among the list of silicon-bonded monovalent hydrocarbon radicals that can be present on the copolymer are halogenated radicals such as 3,3,3-trifluoropropyl. The polydimethylsiloxane oils contain less than one mol percent of vinyl radicals and preferably exhibit a viscosity of from 0.5 to 25 Pa.s.
An objective of the present invention is to improve the physical properties, particularly tear strength and elongation, of high consistency organosiloxane compositions. It has now been found that this objective can be achieved by including in the composition a vinyl-containing liquid polydiorganosiloxane that is immiscible with the gum-type polydiorganosiloxane comprising the major organosiloxane ingredient of the composition. Contrary to the teaching in the aforementioned patent to Simpson, the method used to cure the composition has a profound effect on the physical properties of the cured elastomer.